The invention relates to microwave heating generally and, more particularly, to heating coal in a microwave-energized drying chamber to reduce the coal's moisture content.
Mechanical or thermal drying systems are used to reduce the moisture content of coal prepared and cleaned with water. Reduced moisture content means lower weight, improved handling, and higher furnace efficiency. Gas- or coal-fired ovens, which are conventionally used to dry coal, have a significant fire risk. By heating the exterior surface of a mass of coal, these conventional ovens cause the exterior surface to have a higher temperature than the corresponding interior of the coal. Simultaneously, the conventional ovens also heat the coal as well as the retained water. If the temperature of the coal is raised beyond a specific value, its coking qualities, in the case of metallurgical coal, will begin to deteriorate through oxidation. Microwaves have been used to dry coal because, at microwave frequencies, microwave energy preferentially heats the retained water instead of the coal. But, if the microwave energy is not properly controlled, microwaves can overheat the coal, which affects the coking qualities of metallurgical coal or causes combustion in thermal coal.
Thus, there is a need for a method to dry coal without adversely affecting the coking qualities of metallurgical coals or starting the combustion process in thermal coals.